On Tue, Apr 7, 2009 at 11:58 PM, M.-A. Lemburg <mal@egenix.com> wrote:
This means your proposal actually doesn't add any benefit over the status quo, where you can have an __init__.py that does nothing but declare the package a namespace. We already have that now, and it doesn't need a new filename. Why would we expect OS vendors to start supporting it, just because we name it __pkg__.py instead of __init__.py?
I lost you there.
Since when do we support namespace packages in core Python without the need to add some form of magic support code to __init__.py ?
I think P. Eby refers to the problem that most packaging systems don't like several packages to have the same file - be it empty or not. That's my main personal grip against namespace packages, and from this POV, I think it is fair to say the proposal does not solve anything. Not that I have a solution, of course :) cheers, David
My suggestion basically builds on the same idea as Martin's PEP, but uses a single __pkg__.py file as opposed to some non-Python file yaddayadda.pkg.
Here's a copy of the proposal, with some additional discussion bullets added:
""" Alternative Approach: ---------------------
Wouldn't it be better to stick with a simpler approach and look for "__pkg__.py" files to detect namespace packages using that O(1) check ?
This would also avoid any issues you'd otherwise run into if you want to maintain this scheme in an importer that doesn't have access to a list of files in a package directory, but is well capable for the checking the existence of a file.
Mechanism: ----------
If the import mechanism finds a matching namespace package (a directory with a __pkg__.py file), it then goes into namespace package scan mode and scans the complete sys.path for more occurrences of the same namespace package.
The import loads all __pkg__.py files of matching namespace packages having the same package name during the search.
One of the namespace packages, the defining namespace package, will have to include a __init__.py file.
After having scanned all matching namespace packages and loading the __pkg__.py files in the order of the search, the import mechanism then sets the packages .__path__ attribute to include all namespace package directories found on sys.path and finally executes the __init__.py file.
(Please let me know if the above is not clear, I will then try to follow up on it.)
Discussion: -----------
The above mechanism allows the same kind of flexibility we already have with the existing normal __init__.py mechanism.
* It doesn't add yet another .pth-style sys.path extension (which are difficult to manage in installations).
* It always uses the same naive sys.path search strategy. The strategy is not determined by some file contents.
* The search is only done once - on the first import of the package.
* It's possible to have a defining package dir and add-one package dirs.
* The search does not depend on the order of directories in sys.path. There's no requirement for the defining package to appear first on sys.path.
* Namespace packages are easy to recognize by testing for a single resource.
* There's no conflict with existing files using the .pkg extension such as Mac OS X installer files or Solaris packages.
* Namespace __pkg__.py modules can provide extra meta-information, logging, etc. to simplify debugging namespace package setups.
* It's possible to freeze such setups, to put them into ZIP files, or only have parts of it in a ZIP file and the other parts in the file-system.
* There's no need for a package directory scan, allowing the mechanism to also work with resources that do not permit to (easily and efficiently) scan the contents of a package "directory", e.g. frozen packages or imports from web resources.
Caveats:
* Changes to sys.path will not result in an automatic rescan for additional namespace packages, if the package was already loaded. However, we could have a function to make such a rescan explicit. """
-- Marc-Andre Lemburg eGenix.com
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